The angle is the easy half
A ramp is the hypotenuse of a right triangle. The rise is the deck height, the ramp length is the hypotenuse, and the angle is the arcsine of one divided by the other. A 24 inch deck with an 8 foot ramp gives 14.5 degrees, and doubling the ramp length to 16 feet drops it to 7.2. That relationship is close to linear over the range that matters, which makes it easy to plan: to halve the angle, double the length.
The grade percentage is a different expression of the same thing — rise divided by horizontal run rather than divided by ramp length — and at shallow angles the two are nearly identical. They diverge as the ramp steepens, which is why a 45 degree ramp is a 100 percent grade rather than a 50 percent one.
What the angle actually governs is how much of the vehicle weight is trying to roll it back down and how much traction is left to move it forward. That matters, and it is why a wet aluminium ramp at 15 degrees is a different proposition from a dry one. But it is not usually what stops the loading.
What actually stops a low vehicle
There are three separate geometric problems and only one of them is the ramp angle.
The bottom of the ramp. As the front wheels reach the ramp, the front bumper has to pass over the ramp surface. Whether it does is the vehicle approach angle — the arctangent of the height of the lowest front point divided by its distance ahead of the front axle. A car with a 6 inch air dam 36 inches ahead of the axle has an approach angle of about 9.5 degrees, and it will touch on any ramp steeper than that. This is the contact that people hear and it makes an expensive noise, but it is usually cosmetic.
The top of the ramp. This is the one that actually beaches vehicles. Where the ramp meets the trailer deck there is a corner, and the vehicle has to straddle it — front wheels on the deck, rear wheels still on the ramp, and the whole underside passing over the apex. The vehicle breakover angle describes what it can straddle, and it is twice the arctangent of the lowest central clearance divided by half the wheelbase. A car with 5 inches of clearance and a 100 inch wheelbase has a breakover angle of about 11.4 degrees, which is less than the 14.5 degree ramp above. It gets its front wheels onto the deck and then sits on its crossmember with the drive wheels off the ground.
The bottom of the deck. The same corner exists in reverse when the trailer is on a slope, in a driveway with a lip, or when the tow vehicle squats and the trailer noses down. All of these change the effective angles while the loading is in progress.
Reading the two numbers together
| Situation | Which angle binds | Usual fix |
|---|---|---|
| Lowered car, long wheelbase | Breakover at the top | Longer ramps, or blocks under the ramp ends to soften the break |
| Car with a front splitter | Approach at the bottom | Boards under the ramp toe to flatten the entry |
| Truck or SUV | Usually neither — traction and ramp rating instead | Check the ramp load rating and the surface |
| Mower, ATV, garden equipment | Breakover, and sometimes wheelbase against ramp width | Longer ramps; check the machine deck clearance |
| Non-running vehicle being winched | All of them, plus the pull force | Longer ramps and a plan for the force involved |
The clearance figure that goes into the breakover calculation is the lowest point between the wheels, which is not always the obvious one. On a modern car it is often an exhaust heat shield or a plastic undertray rather than the sump. On a lowered car it is frequently the side skirt, and on a pickup with a long bed it can be the fuel tank or a crossmember well behind the cab. Get under it with a tape measure rather than guessing, because two inches of error changes the answer by several degrees.
Ramps as equipment
Ramps carry ratings, and the rating means something specific that people misread. A pair sold as 3,000 lb capacity is usually rated per pair rather than per ramp, and that rating assumes both ends fully supported: the top hooked or seated properly on the deck, the bottom flat on firm level ground. A ramp resting on grass, gravel or a broken driveway edge, or hooked over only part of the deck lip, is not the ramp that was rated. Ramps also flex under load, which increases the angle at the top exactly when the vehicle is straddling the corner.
Two habits are worth having. Look at where the ramps are standing before anything drives on them, and never stand behind or beside a vehicle going up a ramp — a vehicle that comes off a ramp goes sideways and it does it faster than anyone moves. Once the load is on, its position on the deck determines the tongue weight, which is the tongue weight and load placement calculator, and how it is held down is the cargo securement calculator. For the pull force on something that will not drive up under its own power, see the winch and pulling force calculator. Generic angle and grade conversions, if that is all you need, are on the slope and grade calculator.
Questions people ask
How long should a ramp be for a low car?
Work backwards from the breakover angle rather than picking a length. Measure the lowest point between the wheels and the wheelbase, and the vehicle breakover angle is twice the arctangent of that clearance divided by half the wheelbase. Then the ramp has to be long enough that its angle is comfortably below that figure, with a couple of degrees in hand for ramp flex and for the trailer settling as the car comes on. For a car with 4 inches of clearance and a 105 inch wheelbase, the breakover angle is around 8.7 degrees, and a 24 inch deck needs roughly 13 feet of ramp to stay under it. That is a long ramp, which is why low cars are usually loaded on a tilt-deck or a drop-deck trailer rather than on ramps. Softening the transition with blocks under the ramp ends buys a couple of degrees and is the practical alternative.
What is the difference between breakover angle and ramp angle?
Ramp angle is a property of the ramp: how steeply the surface rises. Breakover angle is a property of the vehicle: the sharpest ridge it can pass over without the middle of the underside touching, determined by ground clearance at the lowest central point and by the wheelbase. They meet at the top of the ramp, where the ramp surface and the trailer deck form a ridge with an included angle set by the ramp angle. If the vehicle breakover angle is smaller than the ramp angle, the vehicle grounds out there — typically with the front wheels on the deck and the rear wheels still on the ramp, which also means the driven wheels may have no weight on them. Long wheelbases make breakover angles worse for a given clearance, which is why a long-wheelbase pickup with plenty of ground clearance can still scrape where a short car does not.
Do I measure the deck height hitched or unhitched?
Hitched, loaded the way it will be when you load it, and with the tow vehicle attached. A trailer sitting on its jack unhitched is at a different attitude from one coupled to a truck, and a truck that squats under tongue weight lowers the deck at the front and raises it at the back. On a trailer with the ramps at the rear, the height that matters is the rear deck height in the hitched, settled state, and it can differ from the unhitched figure by a couple of inches. Two inches on a 24 inch deck is a difference of over a degree on an 8 foot ramp, which is enough to matter when the vehicle breakover angle is marginal. Measure it in the configuration you will actually use.
Can I make a ramp out of lumber?
People do, and the failure mode is not gradual. A plank that carries a walking person is nowhere near carrying a vehicle wheel, timber strength varies with species, grade, moisture, knots and grain in ways that are not visible from the surface, and the span between the ground and the deck edge is exactly where bending stress peaks. A ramp that fails does so under a wheel with a vehicle balanced on it and a person nearby. If you are going to build rather than buy, the design belongs with someone who can size it properly for the load, the span and the actual material — not with a rule of thumb. Manufactured ramps carry a stated rating and a stated way of using them, which is what you are paying for.
Why does the vehicle lose traction halfway up the ramp?
Because weight transfers rearward on a slope, and because at the transition the vehicle can end up straddling the top corner with its driven wheels barely loaded. A front-wheel-drive car climbing a ramp is unloading the very axle that has to pull it, and the moment the front wheels reach the deck while the rears are still on the ramp is when the geometry is worst. Adding to this, ramp surfaces are often smooth metal, and wet or oily metal on a slope offers very little. The practical responses are a longer ramp so the angle is lower, a surface with real grip, loading the vehicle in the direction that puts the driven axle in the better position, and winching rather than driving when the vehicle is marginal. What is not a response is more throttle, which is how vehicles come off the side of ramps.